Coal gasification is the process to convert carboneous materials such as coal into syngas or synthesis gas primarily containing carbon monoxide (CO) and hydrogen (H2). The Synthesis Energy Systems fluidized bed gasifier (“SES gasifier”), described in e.g. U.S. patent application Ser. No. 13/532,769, is commonly used for the gasification of carboneous materials.
FIG. 1 illustrates the basic structure of an SES fluidized bed gasifier, which comprises a vessel housing a headspace 2 above a fluidized bed 1 of the solid material being gasified, and below the bed a conical perforated gas distribution grid 7 through which the gasifying medium is introduced at sufficient velocity to fluidize the solid feed material in the gasifier. Carbonous feed stock, usually as small particles, is delivered just above the gas distribution grid 7. Steam and oxidant (either air or oxygen) are delivered from under the gas distribution grid 7 to fluidize and partially oxidize the solid feed stock.
Immediately underneath the grid, and above other structures, which are usually one or more additional vessels, for ash cooling and processing etc., is an empty “plenum” space 4. The gasifying medium (steam and/or oxygen) is introduced into the gasifier from the plenum space 4 through the gas distribution grid 7.
A passage such as a pipe 6 (“center jet pipe”) in the center region at the bottom of the grid cone introduces oxidant with diluting gas to the bed. Gas velocity of the centre jet pipe 6 is normally greater than the average superficial velocity of gas in the fluidized bed 1. An ash discharge device 5 comprising an annular passage is provided around this center jet pipe for coal ash agglomerated withdrawal and also for provision of additional gas, such as steam, which may serve to cool and protect the center jet pipe 6. The ash discharge device 5 is often configured to comprise a venturi 3 device for sorting the ash particles at the upside of the passage.
The high velocity center jet is an important feature of the existing gasifier as shown in FIG. 1. The gas stream provided through the center jet pipe to the fluidized bed contains a higher oxygen concentration than oxygen concentration provided through the gas distribution grid. This higher oxygen concentration causes more oxidization of the feed stock in the center region of the fluidized bed, and thus the center region has a higher temperature compared to the rest of the bed.
FIG. 2 is a top view of the inside bottom of the vessel, showing an annular space 1 in the center of the conical gas distribution grid 3 formed as the ash discharge pipe through which ash falls down out of the gasifier, and the inlet holes of the centre jet pipe 60 through which gas stream is provided to the fluidized bed.
It is generally desired for the gasifier to be operated at as high a capacity as possible. Increasing the rate at which the feed stock is delivered to the fluidized bed region is the most common way to increase gasifier capacity. Increased feedstock “intake” results in increased fluidized bed depth, and correspondingly, the amount of steam and oxidant needed also must be increased, to both keep the bed fluidized and to ensure sufficient oxidation occurs so as to maintain the operation temperature of the gasifier.
However, due to the limitation of the gas throughput of the gas distribution grid holes, more fluidized gas needs to be delivered through the center jet pipe. As a consequence, the center high temperature region may have a higher than optimal temperature, resulting in excessive ash agglomeration, large ash particles which disrupt ash discharge from the gasifier. In order to prevent such excessive ash agglomeration, the amount of gas delivered through the center jet pipe must be lowered, but this leads to a lowering the operating temperature of the gasifier which would in turn lead to lower conversion rate of the feed to the gasifier.
Furthermore, increasing amount of gas delivery through the center jet pipe necessarily means that the gas velocity is increased, resulting in a disproportionally larger center jet penetration depth. If this penetration depth is larger than the bed depth, the gas will not be able to mix effectively with the particles in the bed, lowering the gasification efficiency.
Another problem facing the above gasifier is the repair or maintenance of the fluidized bed grid and the center jet pipe 6. To repair or maintain this center jet pipe 6, the ash discharge device 5 must first be removed which is difficult because of the additional vessels connected below the ash discharge device.
Therefore, there is a need to have a system and a method to solve the problems of the center jet pipe, without loses of gasifier performance and maintenance.